1. Technical Field
This invention relates generally to an apparatus for and method of moving a slider mounted on mating zipper elements during the production of plastic bags or the like, and in particular to an apparatus for and method of fusing a length of the mating zipper elements and positioning the slider with respect to the fused length.
2. Background Art
The manufacture of thermoplastic bags and the like containing mating zipper elements (i.e., a thermoplastic zipper typically having a rib and groove construction) that are operated by a slider mounted thereon are known in the art. There are a wide variety of designs for such devices. For example, U.S. Pat. Nos. 5,067,208; 4,262,395; and 5,283,932, show different types of sliders and mating zipper elements used in the construction of re-sealable thermoplastic bags. In these and other known arrangements, the sliders operate to connect and disconnect (depending on the direction of movement) opposing mating zipper elements (i.e., an opposing rib and groove) as the slider is moved along the length of the zipper by a user. Thus, by moving the slider, the user can open or close the re-sealable thermoplastic bag or other such product.
In the manufacture of such thermoplastic bags, for example, a web of folded thermoplastic film is used to form a series of bags. Formed on ends of the folded thermoplastic film opposite the fold are corresponding mating zipper elements. As the film is fed along different stages of the manufacturing process, various assemblies perform tasks such as mounting the sliders on the zippers corresponding to separate bags, severing and sealing the film in directions perpendicular to the fold to form the lateral edges of the separate bags, and fusing specified lengths of the mating zipper elements to form the ends thereof for separate bags.
In a typical manufacturing process, the thermoplastic film is indexed to a registered position (i.e., one stage of manufacturing) at which movement of the film is halted while individual components perform different manufacturing processes, such as those described above. At some stages, it may be necessary to move the slider along the mating zipper elements so that it does not interfere with certain processes, or so that the slider is properly positioned for packaging and shipping.
For instance, one stage of the manufacturing process will typically include clamp/stomp members that clamp/stomp a length of the mating zipper elements to meld or crush the elements together to define the ends of the zippers of different bags. Typically, such stomp members are heated so as to fuse the mating zipper elements. It is preferable to move the slider along the zipper after fusion so as to move the slider to a predetermine position along the mating zipper elements.
In one known system for positioning the sliders as described above, the thermoplastic film is indexed to a registered position at which heated stomp members fuse a length of the mating zipper elements. As the fused length is then indexed to the next stage in the manufacturing process, a projection is brought into range of the thermoplastic film such that the projection does not interfere with the movement of the film, but prevents a corresponding slider from moving in the path of the film. This causes the slider to remain still as the film is indexed. The movement of the slider along the mating zipper elements connects/closes the mating zipper elements. Once the film has moved with respect to the slider and projection such that the slider comes into contact with a corresponding fused length of the mating zipper elements, the projection is brought out of contact with the slider. Consequently, the slider is positioned at a predetermined position along the mating zipper elements relative to the fused length of the zipper elements, and resumes movement in the film path.
Of course, this projection arrangement can be used in conjunction with other stages of manufacturing to position the slider for other reasons. For instance, the slider may be moved so as to not interfere with other components of the manufacturing process that act on the film.
However, use of the movable projection discussed above has several drawbacks. In particular, because the projection is operated separately from the apparatus for fusing the mating zipper elements (or other component of the manufacturing process) the timing between the two systems can become out of sync. This in turn leads to incorrect positioning of the slider.
In addition, when the indexing speed of the film is increased, the reliability of the movable projection for precisely positioning the slider degrades. Specifically, at increased manufacturing speeds, the positioning of the projection must take place within a smaller window of time because the point along the mating zipper elements to which the slider must be moved approaches more quickly. Accordingly, small timing errors are magnified at faster operating speeds.
Such inaccuracies can lead to the projection not releasing from the slider in time, causing the slider to move past the intended position or, when the slider is to abut a fused length of the zipper, to catch on the fused portion and build up tension in the moving film. Also, when the slider is being moved to close the mating zipper elements, if the projection releases its contact with the slider too soon or fails to catch the slider at all, the slider may not be moved to the predetermined position relative to the fused length of the zipper.
These problems become even more apparent in modern manufacturing processes in which the film is not indexed between stages, but instead is continually fed through the manufacturing system. In such systems, the film is continually moved along the manufacturing path as movable apparatuses for performing certain manufacturing steps are moved therewith at the same rate. The different apparatuses are typically moved along a rotating path so that they come into contact with the film at given intervals.
For instance, the stomp members may be provided on a rotating belt such that the stomp members move along the film path for a set interval at the same rate of speed as the film (see FIG. 6 and the accompanying text below). Accordingly, the stomp members may stomp and fuse the mating zipper elements as the film and stomp members move together along the film path. Such a manufacturing process allows for much faster run times.
Therefore, not only is there a need for a mechanism to accurately position the sliders in assembly lines running at higher rates of speed, but there is also a need for positioning mechanisms that can operate in continuous movement systems, in conjunction with movable assembly stages such as the moving stomp members discussed above.